1. Field of the Invention
The present invention relates to a conductive material formulation, and particularly to a conductive material formulation used in a solid capacitor. The present invention also relates to a solid capacitor using the conductive material formulation.
2. Description of the Related Art
Capacitors are a type of electronic elements widely used in various electronic products. With the development of technologies, the electronic products tend to being miniaturized and lightened. Therefore, the capacitors used in such electronic products are expected to be miniaturized and have a high capacity and a low impedance at high frequency.
In terms of the types of electrolyte, capacitors are classified into the conventional liquid capacitors and the newly developed solid capacitors. In the prior art aluminum liquid capacitor, a liquid electrolyte was used as a charge transfer substance. The main components of the liquid electrolyte include alcohols with high boiling points, ionic liquid, boric acid, phosphoric acid, organic carboxylic acid, ammonium salt, high polar organic solvent, and a small amount of water. In addition to serving as charge transfer substances, the aforementioned components also have a function of patching an alumina dielectric layer on an aluminum foil. When the aluminum metal in inner layer is exposed due to the defects on the alumina dielectric layer, the patching of the alumina dielectric layer would be achieved by the alumina produced from the reaction of the electrolyte with the exposed aluminum metal during the process of charging and discharging of the capacitors. However, although the cost for conventional aluminum liquid capacitors is lower, the use of liquid electrolyte in said capacitors gives rise to disadvantages such as low conductivity and poor high temperature resistance. Moreover, during the generation of alumina, hydrogen is also generated. The accumulation of excessive hydrogen in the capacitor would cause capacitor rupture and damage the electronic products. Although a hydrogen absorbing agent may be added to the liquid electrolyte to reduce the risk of capacity rupture, the problem is not fundamentally eliminated. The application of conventional liquid electrolyte is limited due to its high equivalent series resistance (ESR) regardless of its high capacity.
In view of the foregoing, a new generation of solid capacitor is developed, in which the liquid electrolyte is replaced by a solid electrolyte. Conductive polymers are one of the developed solid electrolyte. The conductivity of the conductive polymers comes from the holes created by blending the anions of an oxidant into the structure of the polymer as a dopant. Compared with a liquid electrolyte or a solid semiconductor complexing salt such as tetracyanoquinodimethane (TCNQ) complex salt and inorganic semiconductor MnO2 used in a conventional electrolyte capacitor, the conductive polymer has higher conductivity and a suitable high-temperature insulation property. Therefore, the conductive polymer has led the trend of developing the solid electrolyte for the use in current electrolyte capacitors.
In addition to a longer life time which is 6 times longer than that of a general capacitor, solid capacitors also have improved stability and capacitances which does not tend to be influenced by the ambient temperature and humidity. Additionally, the solid capacitor has advantages such as a low ESR, a low capacitance variation rate, an excellent frequency response (high frequency resistance), a high temperature resistance, and a high current resistance, and the problem of leakage and plasma explosion is eliminated.
Jesse S. Shaffer et al disclose a method of using a conductive polymer in an electrolyte of an electrolytic capacitor for the first time in U.S. Pat. No. 4,609,971. The method includes immersing an anode aluminum foil of a capacitor in a mixture solution formed by a conductive polymer polyaniline powder and a dopant LiClO4, and then removing a solvent on the aluminum foil. Due to the excessively great molecular size of polyaniline, it cannot penetrate into the micropores of the anode foil. The impregnation rate of the capacitor obtained through this method is poor, and the impedance is high. In order to facilitate the penetration of the polymer into the micropores of the anode foil, Gerhard Hellwig et al disclose a chemical oxidation polymerization method of using a conductive polymer as the electrolyte of a capacitor in U.S. Pat. No. 4,803,596. The method includes respectively immersing a capacitor in a solution of a conductive polymer monomer and an oxidant, and polymerizing the conductive polymer monomer under a suitable condition, in which the conductive polymer electrolyte is accumulated to a sufficient thickness through multiple immersions. Thereafter, Friedrich Jonas et al. of the Bayer Corporation in Germany disclose a method of manufacturing an aluminum solid capacitor with poly-3,4-ethylenedioxythiophene (PEDOT) as an electrolyte by using a 3,4-ethylenedioxythiophene (EDOT) monomer in combination with an oxidant, iron (III) p-toluenesulfonate, for the first time in U.S. Pat. No. 4,910,645. In addition, it is found that 3,4-ethylenedithiathiophene (EDTT) which has a similar structure with EDOT may be transformed into an electrically active polymer (Lambertus Groenendaal et al. Adv. Mater. 2000, 12, No. 7).
The conductive polymer PEDOT has advantages such as a high heat resistance, a high conductivity, a high charge transfer velocity, being toxicity-free, a long lifetime, and no occurrence of capacitor rupture when being applied to a capacitor. PEDOT is prepared by performing the polymerization of EDOT monomer and iron p-toluenesulfonate directly in capacitors. Said process belongs to an in situ reaction and may be performed by a one-liquid method, a two-liquid method or a multi-liquid method classified in terms of the immersion process. The one-liquid method includes immersing capacitor elements in a mixed solution of EDOT and iron p-toluenesulfonate prior to a heat polymerization. The two-liquid method includes immersing capacitor elements respectively in EDOT and iron p-toluenesulfonate prior to a heat polymerization. However, in the one-liquid method, careful control of the process parameters is required so as to prevent EDOT from polymerization before the immersion, and the two-liquid method subjects to solution contamination problems.
In addition, the PEDOT resulting from the in situ polymerization on the surface or in the pores of the dielectric layer on the anode foil has a powder structure, and such powder structure tends to have a low degree of polymerization, poor physical properties, easy shedding due to poor adhesion on the electrode surface or surface pores and limited tolerant operating voltage. Due to such disadvantages of the powder structure of PEDOT, a solid capacitor of 16 V or more cannot be achieved.
To solve the foregoing problems, Stephan Kirchmeyer et al. proposed a non-w situ polymerization to synthesize conductive polymers (J. Mater. Chem. 2005, 15, 2077-2088), however, the conductive polymer synthesized by non-w situ polymerization generally has disadvantages such as short repeating units (about 6 to 18 repeating units) and a low degree of polymerization (a weight average molecular weight of about 2500 or less). Due to the poor physical properties resulting from a low degree of polymerization, polymers with a low degree of polymerization cannot be used for a high voltage operational environment.
Therefore, in order to satisfy the requirements of miniaturization, high capacity, high temperature resistance and high frequency, a conducting material which has improved physical properties and can be applied to solid capacitors with a higher voltage resistance and better stability is expected to be a substitute for conventional liquid capacitors.
Provided is a solid capacitor having a solid electrolyte with improved physical properties to reduce the chance of electron breakthrough, enhance the insulation between the cathode and anode in the solid capacitor and improve the withstand voltage of the solid capacitor.